EP0742453A1 - Wideband high reflectivity mirror and its manufacturing process - Google Patents

Abstract

he mirror, having a wide band and high level of reflectivity, consists of a substrate covered with alternating layers of materials with two different refractive indices, one of which is higher than the other. The alternating layers are covered by a surface layer of a 3rd material which has a refractive index carrying with a continuous profile, either periodic or sinusoidal. The substrate is made from silica, quartz or glass and a layer of metal, esp. AL, is incorporated between the substrate and alternating layers to increase the mirror's reflectivity. The alternating layers are of SiO2 and TiO2, and the surface layer is of silicon oxynitride. The layering is carried out by ion beam pulverisation.

Description

TECHNICAL AREA

The present invention relates to a broadband mirror with high reflectivity.

It relates more precisely to mirrors which can be used both in normal incidence and in oblique incidence with angles greater than 45 °.

Depending on their spectral range, these mirrors are used, for example, as broadband mirrors for a tunable laser. They can also be used in high flux lasers or in devices with several wavelengths.

STATE OF THE PRIOR ART

The problem which the invention proposes to solve is that of obtaining a mirror which has both an extended reflection band, good reflectivity, and good resistance to light flux.

The expression “reflection band” of a mirror is understood to mean the width of the wavelength (or frequency) spectrum for which the reflectivity, that is to say the ratio between the intensity of the reflected flux and the intensity of the incident flux is greater than a determined value.

Good reflectivity is obtained in known manner by stacking an alternation of layers of dielectric materials of high and low refractive indices n ₁ and n ₂ with n ₂ > n ₁ and having an optical thickness equal to a quarter of the central wavelength of the mirror strip. Such a stack is commonly referred to as a Bragg mirror. It is preferably carried out on a transparent substrate.

Document (1) relates to different types of mirrors and in particular to mirrors formed by a alternating stacking of layers. Like all the documents cited below, it is mentioned at the end of the description.

où n_a est l'indice de réfraction de l'air et N le nombre de paires de couches de haut et bas indice (n₁, n₂).According to this document, the reflectivity R for a Bragg mirror is expressed as follows: $$\text{R =}{\left(\left[\frac{\text{1-}\left(\frac{{\text{not}}_{\text{s}}}{{\text{not}}_{\text{at}}}\right){\left(\frac{\text{not}{}_{\text{2}}}{\text{not}{}_{\text{1}}}\right)}^{\text{<figure-callout id="1" label="2N" filenames="imgf0003.png,imgf0004.png" state="{{state}}">2N</figure-callout>}}}{\text{1+}\left(\frac{{\text{not}}_{\text{s}}}{{\text{not}}_{\text{at}}}\right){\left(\frac{\text{not}{}_{\text{2}}}{\text{not}{}_{\text{1}}}\right)}^{\text{<figure-callout id="2" label="2N" filenames="imgf0001.png,imgf0003.png" state="{{state}}">2N</figure-callout>}}}\right]\right)}^{\text{2}}$$

where n _a is the refractive index of air and N the number of pairs of layers of high and low index (n ₁ , n ₂ ).

The width of the band Δλ of such a mirror is expressed as follows: $${\text{Δλ = λ}}_{\text{vs}}\text{·}\frac{\text{2}}{\text{π}}\text{·}\frac{{\text{Δ}}_{\text{not}}}{\text{<not>}}$$ where λ _c is the central wavelength, Δ _n the difference n ₂ -n ₁ of the indices and 〈n〉 the average index between the two materials.

It appears that the width of the strip is essentially linked to the difference between the high index and the low index.

The most commonly used materials for such mirrors are: silicon oxide (SiO ₂ ), titanium oxide (TiO ₂ ), hafnium oxide (HfO ₂ ) and zirconium oxide (ZrO ₂ ); SiO ₂ being used as a low index material.

Among these materials, TiO ₂ has the highest refractive index (n = 2.45) and is therefore used with SiO ₂ for the production of broadband mirrors.

Unfortunately, as explained in document (2), the mirrors produced with the pair of TiO ₂ -SiO _{2 materials} do not have very good resistance to laser flux, compared to the mirrors produced with the HFO ₂ -SiO ₂ couples or ZrO ₂ -SiO ₂ . TiO ₂ is therefore ruled out.

The conditions of wide band and good reflectivity appear to be difficult to reconcile with a satisfactory resistance to intense laser fluxes.

One solution to the problem, mentioned in document (3) on pages 175 and following, consists in a known manner of stacking two Bragg mirrors, the mirrors being centered on two offset wavelengths λ ₁ and λ ₂ . The wavelengths λ ₁ and λ ₂ are chosen close enough not to introduce any significant discontinuity in the reflection band of the resulting mirror. This strip is somehow the sum of the reflection strips of the two individual mirrors. The reflection bands of the individual mirrors can therefore be a little narrower, which makes it possible to use the pairs of materials having a smaller index difference but having better resistance to flow.

Such a mirror is not entirely satisfactory. In addition to problems of reduced reflectivity in the portion of overlap of the spectral bands of the two Bragg mirrors, such a structure suffers from the technological difficulty of its production. Indeed, the stacking of two mirrors results in a substantial increase in the thickness of the structure. The stack which can reach more than 5 μm for a reflectivity mirror greater than 99.5% is very fragile. In addition, internal mechanical stresses further increase the risk of destruction of the structure. Given the limited choice of materials to make the stacks, it is difficult to also meet the mechanical strength requirements. The production yield is reduced and the cost of the double mirrors becomes too high.

Furthermore, and as mentioned above, the stacking of the two Bragg mirrors forms a sort of Perot-Fabry type cavity which generates a significant transmission peak in the spectral band, in particular for wavelengths located at 1 intersection of the reflection bands of the individual mirrors. Finally, a double mirror structure is not a satisfactory solution.

où λ_o est la longueur d'onde de centrage de la bande du miroir et δλ un écart autour de λ_o calculé pour chaque couche.To obtain a broadband mirror, yet another possibility, also mentioned in document (3), consists in making a stack of which each layer has an optical thickness equal to

where λ _o is the centering wavelength of the mirror strip and δλ a deviation around λ _o calculated for each layer.

Such a mirror is difficult and expensive to produce, in particular because of the need to determine δλ for each layer, the need for very precise control of the thickness of the layers, and because of the large number of layers which is essential to obtain a suitable reflectivity over the entire spectral band of the mirror.

Finally, it appears that it is very difficult to obtain mirrors suitable for example for tunable lasers with intense flux, for a reasonable manufacturing cost.

An object of the present invention is to provide a mirror with high reflectivity, wide bandwidth and good resistance to laser light flux which does not have the drawbacks of the mirrors of the prior art mentioned in the foregoing.

Another object is to propose a method for producing such a mirror.

STATEMENT OF THE INVENTION

The mirror of the invention combines a mirror composed of alternating discrete layers of index n ₁ and n ₂ and a layer of continuously variable index.

To this end, the mirror of the invention comprises a substrate, a stack placed on the substrate and formed by alternating layers of a first material having a first refractive index and layers of a second material having a second refractive index greater than the first refractive index, and a layer of a third material disposed on the stack, said layer having a refractive index which varies according to a continuous profile. The layer of the third material therefore does not have "index hops" from one value to another, such as stacking.

By adapting the index profile of this layer to the conditions of use of the mirror, it is possible not only to increase the reflection band and the reflectivity of the mirror, but also to significantly reduce the electric field of the penetrating light flux. in the stack of discrete layers. According to one aspect of the invention, the profile of the index is chosen to be periodic, for example sinusoidal. The calculation of the refractive index profile is obtained for example with a so-called annealing and entropy minimization algorithm, detailed in the document (4).

In this algorithm, the function Ci represents the difference between the calculated optical properties and the optical properties sought for a given configuration i of parameters.

ΔCi being a variation of this function after an elementary disturbance.

After each perturbation, the optical properties can be calculated by a matrix model for homogeneous isotropic layers with a parallel plane face. The principle of this calculation is given in detail for example in document (1).

For this calculation, the continuously variable index layer is broken down into discrete layers. For example, a sinusoid period can be broken down into 30 elementary layers with which characteristic matrices are associated. The product of these matrices and matrices associated with the Bragg mirror formed by the stack gives the optical properties of the entire structure. For these calculations, we will usefully refer to document (5).

Thus, the variable index layer not only makes it possible to widen the spectral band of the mirror and to increase its reflectivity, but also to protect the stack from excessively high electric fields.

The stacking is preferably carried out with the pair of materials SiO ₂ -TiO ₂ chosen because it makes it possible to produce mirrors which have spectral bandwidths greater than those of the mirrors produced with other pairs of materials. The stack can constitute a Bragg mirror.

In this case, the thickness of each of the layers of the stack is equal to λo / 4 where λo is the centering wavelength of the mirror. Thanks to the variable index layer, this stack is subjected to a more moderate flux than the incident flux.

According to an advantageous variant, the mirror also comprises a metal layer disposed between the substrate and the stack. This metallic layer makes it possible to increase the reflectivity of the mirror and consequently to reduce the number of layers of the stack. Aluminum is an example of a metal suitable because of its high reflectivity and the wide spectral reflection band.

Advantageously, the substrate can be made of silica, quartz, glass or ceramic. The choice of substrate can influence the desired spectral response. However, in the case where a metal layer is provided to increase the reflectivity of the mirror, the substrate will be chosen according to other properties, such as its thermal conduction.

• a) - dépôt sur un substrat d'une couche métallique,
• b) - formation sur la couche métallique d'un empilement alterné de couches d'un premier et d'un second matériaux ayant respectivement un premier indice de réfraction et un second indice de réfraction supérieur au premier indice de réfraction,
• c) - formation sur l'empilement d'une couche d'un troisième matériau présentant un indice de réfraction qui varie selon un profil continu.

To make a mirror in accordance with the invention, a process is advantageously used comprising the following steps:

• a) - deposition on a substrate of a metal layer,
• b) - formation on the metallic layer of an alternating stack of layers of a first and a second material having respectively a first refractive index and a second refractive index higher than the first refractive index,
• c) - formation on the stack of a layer of a third material having a refractive index which varies according to a continuous profile.

Alternating stacking is carried out, for example, using an ion beam sputtering technique called IBS (Ion Beam Sputtering). This deposition technique is very advantageous for the deposition of dielectrics because of its energy character. It makes it possible to obtain dense materials, and of high optical index, which are very advantageous in the context of the present application. An illustration is given in particular in document (6).

For the variable index layer, for which a very precise deposit control is required, a beam spraying technique is preferably used. ion, assisted by ion beam, called DIBS (Dual Ion Beam Sputtering).

An example of the DIBS technique is illustrated by the publication of S.J. Holmes in document (7).

This technique, using a silicon target in a reactive oxygen and nitrogen atmosphere, allows fine control of the production of a silicon oxynitride deposit, the index profile of which can be continuously adjusted by modulating the pressure. partial oxygen with respect to partial nitrogen pressure.

BRIEF DESCRIPTION OF THE FIGURES

• FIG. 1 is a schematic section of a mirror according to the invention,
• FIG. 2 represents the index profile of a variable index layer and of an alternating stack, in accordance with one aspect of the invention,
• FIG. 3 graphically represents the spectral band of a known mirror and of mirrors according to the invention,
• Figures 4 to 7 illustrate the parallel and perpendicular components of the electric field of an incident flux, in the mirror of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

The structure of the mirror 1 represented in FIG. 1 comprises a metal layer 2, an alternating stack 4, for example a Bragg mirror, and a variable index layer 6, stacked in this order on a substrate 8. In the embodiment described in As an example, mirror 1 is centered on a wavelength λ _o = 652 nm and has a spectral bandwidth Δλ = 145 nm with a reflectivity R ≧ 99.7% for a flux arriving at 45 ° in polarization P.

The substrate plays no optical role in this embodiment, insofar as it is covered by the metal layer 2. The substrate is made for example of silicon.

For layer 2, metals such as aluminum or silver can be chosen which have the advantage of good reflectivity in the visible and infrared spectrum and an extended spectral band. However, aluminum is preferred to silver since its degradation in contact with air is less rapid.

The deposition of this layer takes place for example by a traditional deposition technique by vacuum evaporation.

The stack 4 is preferably produced by an ion beam spraying technique. In the present example, the stack 4 is a Bragg mirror which comprises seven pairs of alternating layers.

Each pair of layers is formed of a layer 12 of TiO ₂ with a refractive index n ₂ = 2.4 and a thickness of 68 nm and a layer 14 of SiO ₂ with a refractive index n ₁ = 1.48 and 110 nm thick.

Layer 6 with variable index is produced according to the DIBS technique. In the context of the example described, the index profile, chosen sinusoidal, is optimized for the particularly difficult condition of a P polarization at 45 ° (π / 4 radians) of incidence.

Layer 6, made of silicon oxynitride has a refractive index centered on 〈n〉 = 1.8 and varying with an amplitude Δn = ± 0.3 according to a period p = 205 nm and with a phase at the origin φ = 4 radians.

The index n varies in a direction perpendicular to the layer 6 and as a function of a thickness z measured from the surface 20 of the mirror.

The value of the index as a function of z is given by the formula: $$\text{n (z) = 〈n〉 + Δn.sin}\left(\frac{\text{2π}}{\text{p}}\text{z + φ}\right)$$

Figure 2 illustrates the index profile of the mirror.

The index, plotted on the ordinate, is expressed as a function of the thickness z measured from the surface 20, plotted on the abscissa and expressed in nanometers.

It is verified that the refractive index of layer 6 varies sinusoidally between 1.5 and 2.1 with a period of 205 nm. The Bragg mirror is distinguished by discontinuous index values corresponding to n = 1.48 for SiO ₂ and n ₂ = 2.4 for TiO ₂ .

FIG. 3 shows the increase in reflectivity and the widening of the spectral band of the mirror according to the invention.

In this figure, curve 21 gives the reflectivity as a function of the wavelength of a traditional mirror formed of a metallic layer and of a Bragg mirror centered on 652 nm.

Curves 22 and 24 correspond to a mirror according to the invention where the variable index layer of silicon oxynitride (SiON) has been optimized respectively to widen the spectral band towards short and long wavelengths. It can be seen that the width of the strip of the mirrors according to the invention, respectively 150 nm and 160 nm is much greater than the spectral width of the strip of the conventional mirror corresponding to curve 21, which is 105 nm.

Furthermore, the layer of silicon oxynitride of the mirror according to the invention makes it possible to reduce in significant proportions the electric field of the incident flux which enters the Bragg mirror.

To express this field we decompose it into a component Ep parallel to the plane of incidence of the mirror and in a component Es perpendicular to the plane of incidence.

Figures 4 to 7 give the perpendicular S and parallel P components of the electric field as a function of the thickness z of the mirror described in the above and for different wavelengths chosen in the spectral band of the mirror where the reflectivity is greater than 99.7% (680, 625, 600 and 550 nm).

In FIG. 4, the components S and P of the field are respectively represented by reference curves 426 and 428. They correspond to an incident flux at 45 ° having a wavelength on the order of 680 nm.

A first region 430 corresponding to the variable index layer 6 is distinguished in FIG. 4, a second region 432 corresponding to the Bragg mirror 4 where dotted lines 433 recall the layers 12, 14, and a third region 434 corresponding to the layer d 'aluminum. In region 432, we can note discontinuities 436 corresponding to the interfaces between layers of high and low refractive index.

In FIGS. 5, 6 and 7 which always correspond to an incident flux at 45 °, but for wavelengths respectively of the order of 625, 600 and 550 nm, the elements corresponding to those of FIG. 4 carry corresponding references but whose hundreds figure indicates the figure number.

Generally, it can be seen in FIGS. 4 to 7 that, thanks to the variable index layer for wavelengths fairly close to λo, the electric field reaching stack 4 is clearly attenuated compared to the electric field at the surface. . The stack is therefore protected from very intense flows.

THE DOCUMENTS MENTIONED IN THE FOREGOING HAVE THE FOLLOWING REFERENCES:

• (1) OPTICAL WAVES IN LAYERED MEDIA / POSCHI YEH ED. Willey interscience
• (2) Laser conditioning of optical thin films CR WOLFE & al. N.I.S.T. public Boulder dammage symposium, CO USA 1989, p.360-375
• (3) THIN FILM OPTICAL FILTERS HA MACLEOD, ed. Adam hilger (second edition)
• (4) SIMULATED ANNEALING: THEORY AND APPLICATIONS PJM VAN LAARHOVEN 1 EHL AARTS Kluwer academic publishers
• (5) Evaluation and production of dielectric mirrors with a continuous and periodic index profile (rugate filters).
  PhD thesis from Joseph Fourier University - Grenoble 1
  L NOUVFLOT, May 1993
• (6) Production of thin layers of TiO ₂ and SiO ₂ by ion beam spraying CNAM engineering thesis M IDA 1990
• (7) SOCIETY OF VACUUM COATERS DALLAS 1993 conference
  Reactive ion beam sputter deposition of gradded interface optical thin films of aluminum oxynitride and siliconoxynitride SJ HOLMES & al. Northrop corporation

Claims (11)

Broadband mirror with high reflectivity comprising a stack (4) disposed on a substrate (8) and formed by alternating layers (12, 14) of a first material having a first refractive index (n ₁ ) and layers of a second material having a second refractive index (n ₂ ) greater than the first refractive index (n ₁ ), characterized in that it also comprises a surface layer (6) of a third material arranged on the stack, said layer having a refractive index which varies according to a continuous profile. Mirror according to claim 1, characterized in that the refractive index of the surface layer (6) varies according to a periodic continuous profile. Mirror according to one of claims 1 or 2, characterized in that the refractive index of the surface layer (6) varies according to a sinusoidal profile. Mirror according to any one of the preceding claims, characterized in that the substrate (8) is made of a material chosen from silica, quartz and glass. Mirror according to any one of the preceding claims, characterized in that it further comprises a metal layer (2) disposed between the substrate (8) and the stack (4), so as to increase the reflectivity of the mirror (1 ). Mirror according to claim 5, characterized in that the metallic layer (2) is a layer of aluminum. mirror according to any one of the preceding claims, characterized in that the first material is SiO ₂ and the second material is TiO ₂ . Method for producing a broadband and high reflectivity mirror, characterized in that it comprises the following steps: a) - deposition on a substrate (8) of a metal layer (2), b) - formation on the metallic layer of an alternating stack (4) of layers (12, 14) of a first and a second material having respectively a first refractive index (n ₁ ) and a second refractive index (n ₂ ) greater than the first refractive index, c) - formation on the stack of a layer of a third material having a refractive index which varies according to a continuous and periodic profile. Method according to claim 8, characterized in that the stacking is carried out according to an ion beam spraying technique. Method according to one of claims 8 or 9, characterized in that the variable index layer is produced according to an ion beam spraying technique assisted by ion beam. Method according to claim 8, characterized in that the layer of third material is a layer of silicon oxynitride.

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